[8.x](backport #42096) test: acquire lock before checking pendingRequests in controller test

[mergify]

Proposed commit message

fix race condition in TestQueueProducerBlocksUntilOutputIsSet

controller.queueProducer will modify pendingRequests so we need to acquire a lock before checking the slice

data race:

==================
WARNING: DATA RACE
Write at 0x00c0002b02a8 by goroutine 39:
  github.com/elastic/beats/v7/libbeat/publisher/pipeline.(*outputController).queueProducer()
      github.com/elastic/beats/v7/libbeat/publisher/pipeline/controller.go:237 +0x304
  github.com/elastic/beats/v7/libbeat/publisher/pipeline.TestQueueProducerBlocksUntilOutputIsSet.func1()
      github.com/elastic/beats/v7/libbeat/publisher/pipeline/controller_test.go:229 +0x35

Previous read at 0x00c0002b02a8 by goroutine 30:
  github.com/elastic/beats/v7/libbeat/publisher/pipeline.TestQueueProducerBlocksUntilOutputIsSet.func2()
      github.com/elastic/beats/v7/libbeat/publisher/pipeline/controller_test.go:236 +0x38
  github.com/elastic/beats/v7/libbeat/publisher/pipeline.waitUntilTrue()
      github.com/elastic/beats/v7/libbeat/publisher/pipeline/testing.go:178 +0x76
  github.com/elastic/beats/v7/libbeat/publisher/pipeline.TestQueueProducerBlocksUntilOutputIsSet()
      github.com/elastic/beats/v7/libbeat/publisher/pipeline/controller_test.go:233 +0x426
  testing.tRunner()
      testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      testing/testing.go:1743 +0x44

Goroutine 39 (running) created at:
  github.com/elastic/beats/v7/libbeat/publisher/pipeline.TestQueueProducerBlocksUntilOutputIsSet()
      github.com/elastic/beats/v7/libbeat/publisher/pipeline/controller_test.go:228 +0x2f1
  testing.tRunner()
      testing/testing.go:1690 +0x226
  testing.(*T).Run.gowrap1()
      testing/testing.go:1743 +0x44

Goroutine 30 (running) created at:
  testing.(*T).Run()
      testing/testing.go:1743 +0x825
  testing.runTests.func1()
      testing/testing.go:2168 +0x85
  testing.tRunner()
      testing/testing.go:1690 +0x226
  testing.runTests()
      testing/testing.go:2166 +0x8be
  testing.(*M).Run()
      testing/testing.go:2034 +0xf17
  main.main()
      _testmain.go:81 +0x164
==================
--- FAIL: TestQueueProducerBlocksUntilOutputIsSet (0.02s)
    testing.go:1399: race detected during execution of test

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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---

This is an automatic backport of pull request #42096 done by [Mergify](https://mergify.com).

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @kruskall? 🙏